Optical fiber connectors that comprise a glass ferrule are known. See U.S. Pat. No. 4,850,670. However, despite potential cost advantage over conventionally used ceramic ferrules, glass ferrules have found only limited use, e.g., in the so-called rotary splice. This general failure to adopt an otherwise advantageous technology is due at least in part by the failure of many prior art glass ferrules to meet stringent mechanical requirements, including strength and dimensional standards. Indeed, in the rotary splice there is only minimal mechanical stress on the glass ferrule since the rotary splice is designed for one time assembly.
In view of the significant cost savings that can be realized from the replacement of ceramic ferrule optical fiber connectors with relatively inexpensive glass ferrule optical fiber connectors, it would be highly desirable to have available glass ferrules with improved strength that can meet the design standards for current connectors, and also have the dimensional control necessary to meet those standards.
U.S. Pat. No. 5,295,213 discloses a method of strengthening alkali-containing glass ferrules by ion exchange. The ion exchange method applies to borosilicate glass containing 3 to 10 wt. % Na.sub.2 O, and results in a thin layer of strengthened glass on the outer surface of the glass where the ion exchange process occurs. However, this layer is thin, typically 10-20 .mu.m, and is easily abraded away in practical service and the ferrule then returns to its original weak state. Moreover, this technique is not applicable to vitreous silica or PYREX.TM. ferrules. It is well known that glasses with a higher amount of sodium and with a significant amount of alumina are more effective when treated by an ion-exchange process, and we have used such glass compositions, e.g. those described in U.S. Pat. No. 3,661,545 to make ferrules with a strengthened layer 75 .mu.m thick. This ferrule can survive very harsh abrasion treatment and thermal shock with only moderate loss of the enhanced strength.
Another technique for producing glass ferrules for optical fiber connectors is described and claimed in U.S. Pat. No. 5,598,496. This technique involves etching the outer surface of the glass ferrule to improve the strength of the glass, and coating the etched surface with an adherent coating of, e.g. Ni and Au.
In these and other similar known techniques, glass ferrules are produced from a tubular preform by drawing the preform into a continuous glass tube, and cutting the tube into sections each of which becomes a glass ferrule. Since the early recognition of the potential economies of substituting glass ferrules for ceramic ferrules, a concern about reliability of glass ferrule manufacture has been the dimensional control capabilities of glass making technology as compared with the known dimensional precision inherent in ceramic technology. In practice, it has been found that relatively good dimensional control can be realized with glass ferrule fabrication techniques. This is due to inherent behavior of glass during tube drawing in which the geometry of the preform is replicated to a high degree in the tube, and the success of glass ferrule technology so far has relied on that inherent property. While the replication in the drawn tube is indeed high, the ability to manufacture the starting preforms with precise dimensional control is less assured.
Preforms for these processes may be produced e.g. by extruding large cylindrical hollow bore glass bodies with a relatively large outside diameter, and lengths of e.g. 20" or more. It is found in practice that no two extruded preforms are alike in inside and outside diameter and ratio OD/ID, and some preforms have eccentricity between OD and ID. With such variations in preform manufacture it is difficult to produce drawn glass ferrules with required dimensional standards. For multimode fiber ferrule connectors, dimensional control should be within .+-.1.5 .mu.m for OD, and within .+-.3.0 .mu.m for ID. For single mode fibers, the precision required is even higher. In terms of the precision required for preform dimensions, for a preform with e.g. a 40 mm OD, to be drawn into 2.5 mm OD ferrules, the OD of the preform should be controlled to within .+-.24 .mu.m. The standards for ovality and concentricity are equally stringent.
Efforts have been made to adjust the OD, and bring the OD/ID ratio into compliance with desired limits, by mechanically machining the preform. This is a known technique for adjusting the dimensions of glass preforms, and typically involves removing glass material from the preform using a glass lathe. However, this approach does not allow adjusting the ID of the preform, and is not effective in many cases for correcting eccentricity in hollow bore preforms.
Moreover, in the course of this work it was discovered that almost all of the hollow bore cylindrical preforms produced by one of the glass forming methods, i.e. extrusion, exhibit a slight bowing, typically 0.2-1 mm per 750 mm length (approximately 0.02--0.13%). This bowing occurs is essentially congruent in both the OD and the ID along the length of the hollow bore cylinder. Since the bowing occurs in essentially equal measure in both the OD and the ID, it does not affect the ratio OD/ID in the drawn state and therefore is considered to be within the dimensional tolerance of the process. Thus it is an acceptable defect in conventional practice. However, when the OD is adjusted by machining, typically machining in a glass lathe, the outer bow, or a portion thereof, is removed while the inner bow remains. This produces an eccentricity in the preform (or exacerbates existing eccentricity) which varies axially along the preform. This lack of concentricity is replicated in the drawn state. Any significant offset in the location of the center bore is not acceptable in the drawn ferrule connectors.
A process for making glass ferrules with high dimensional stability, and with the capability of adjusting both the ID and OD of the preform without causing bore eccentricity would be a significant advance in this technology.